The present invention relates to an improved hubless castor wheel construction, specifically designed for connection to furniture articles and the like.
The castor wheel according to the invention can be specifically used for easily driving furniture articles or pieces, such as chairs, tables, desks, hand-actuated carriages for transporting materials, such as shopping carriages.
As is known, available prior tilting or castor wheels can be conventionally divided into two types.
The first provides to use a single wheel element including, at the center thereof, a rotary axis cooperating with a supporting element which is in turn coupled to a furniture piece or carriage through a pin allowing the wheel assembly to turn about a substantially vertical axis, thereby allowing the furniture piece or carriage to be displaced according to any desired directions.
The mechanisms of the above mentioned first type, however, have the disadvantage that the wheel assembly thereof encounters a resistance against its rotation about a vertical axis, thereby hindering a smooth movement of the furniture piece or carriage to which the wheel assembly is coupled is hindered.
Actually, to allow the wheel assembly to evenly turn about its vertical axis, the wheel must be capable of rotating about a line passing through the contact point of the wheel and the floor.
On the other hand, a peripheral portion of the wheel tends to drag on the floor, instead of freely turning.
Moreover, the wheel central rotary axis is fixed to the outside of the wheel by supporting elements which, as it is conventional, do not comprise protective means designed for protecting them against possible impacts with objects, such as other furniture pieces or walls, which impacts could damage or offset the wheel rotary axis and prevent an efficient operation of the wheel assembly.
The wheel assemblies of the second wheel type, in turn, comprise two wheel elements having a shared rotary axis, which is typically fixed to the wheel assembly supporting element at a middle position between the two wheels thereof, which wheels, however, are adapted to separately freely rotate, in particular in mutually opposite directions, thereby allowing the wheel assembly to easily turn about a pivot pin fixing the wheel assembly to the furniture piece or carriage, as disclosed for the first type, to further improve the direction switching evenness.
Moreover, in the second type of wheel assembly, the central axis support is arranged at the center of the supporting arrangement and between the two wheels, thereby protecting it against possible impacts and damages, to prevent the operating efficiency from being deteriorated.
However, a problem affecting the second type of wheel assembly, is the fixing system used for fixing or attaching the wheel elements to the central axis.
In fact, differently from the wheel assemblies of the first type, in which the central axis is fixed to the two opposite faces of the wheel, in the wheel assemblies of the second type, each wheel is respectively attached to an opposite end portion of the central axis, the axis support being arranged between the two wheels.
This attachment system is rather inefficient from a stability standpoint: in fact, since the axis does not pass through the wheel, the wheel load on said axis is asymmetrical and, in particular, maximum near the wheel inner part joined to the axis and minimum, or zero, outside of the wheel.
The above mentioned stability lackness generates, as can be easily found by examining a wheel assembly of this type, a comparatively high clearance between the wheels and central axis, which not only causes an inefficient operation of the wheel assembly, but, moreover, negatively affects the construction quality of the overall furniture article or carriage the wheel assembly is applied to.
Moreover, both the above mentioned castor wheel types have yet further drawbacks, since their strength limits or properties directly depend on the assembling procedure thereof.
With respect to the wheel assembly including a single wheel element, the strength limits are due to the fact that the supporting element is usually anchored only to a side of the wheel, with a consequent objectable great inclination of the central axis of the wheel, preventing the latter from properly turning as it impacts against an obstacle.
With respect to the second type of wheel, including two wheel elements having a single shared pivot axis fixed to a central support element, the small strength of the wheel to impacts and wear is due to the assembling system, anchoring one of the two wheels to the central support, and to the other wheel, by a latching or locking system arranged on a side disc element of the opposite wheel susceptible to be anchored to the first.
In the most common prior arrangements, the wheel turns about its pivot axis, the rotary movement being permitted by the very low friction coefficient between the wheel element and axis.
Since the diameter of the wheel with respect to the rotary axis engaging hole is typically very large, then, the torque on the axis-wheel coupling hole will be in turn very great.
Thus, it has been found that, upon an extended use of such a wheel assembly, friction will progressively remove material from the hole, to enlarge the latter, to cause an unstable rotation of the wheel on said axis, and an inefficient operation of the wheel assembly.
Moreover, if the rotary speed of a wheel is a comparatively low and said wheel bears a comparatively large load, the adoption of a central pivot axis does not represent the most efficient approach.
In fact, the rotary wheels provide, as they turn, a torque about a line passing through the center of the wheel and perpendicular to the movement direction, since the force on the wheel, at its floor contact point, is multiplied with its distance from the rotary axis, i.e. the wheel radius.
This torque represents an additional load on the wheel/axis assembly, thereby further decreasing the stability of the wheel assembly.
In this connection it should be pointed out that castor wheels designed for overcoming some of the above mentioned drawbacks have been recently designed.
Such a wheel assembly substantially comprises a supporting element having an inner annular or ring-like portion thereabout turns a toroidal wheel through an interposition of sliding elements.
Said sliding elements, which can comprise either balls or rollers, are arranged in suitable sliding recesses formed both on the inner annular portion and on the toroidal wheel inner portion.
Thus, owing to their front arrangement, they hold inside the wheel assembly the sliding elements to allow them to properly operate.
However, for making the above construction, it is necessary to provide said recesses which must be specifically machined to provide a precise coupling, thereby inevitably increasing the making cost and, consequently, the selling cost of the wheel assembly, with a consequent reduction of the related profit spread.
Moreover, as the above mentioned hubless wheel assembly is used through an extended period of time, the sliding elements tend to damage the contours of said recesses, to enlarge them, thereby increasing the clearance between the inner annular element and toroidal wheel, to consequently further decrease the stability of the wheel assembly.